A good sign is that the people who had the resistant strain have not passed the disease on to other people, said David Reddy, leader of Roche's Tamiflu pandemic task force. And the resistant variety has been like the widespread version of the virus in that it typically produces only mild symptoms.
The 13 cases were scattered around the world in Europe, the United States and Asia, said Reddy.
Tamiflu, whose generic name is oseltamivir, is one of two main antivirals in the arsenal against swine flu as the world awaits the widespread availability of a vaccine against the disease.
Reddy told reporters that the low rate of resistance was in line with tests the company has conducted, which indicated that 0.32 percent of adults and 4 percent of children who took the Roche drug developed resistance to it.
Roche is keeping a close watch on the virus' interaction with the drug, one of the first defenses against the disease, he said.
Reddy told The Associated Press that it wasn't certain why the 13 patients developed the resistance while taking the drug, but that there was an indication that many were taking only half a dose.
That lower dose is what is given to people to prevent them catching the disease, said Reddy. "If they were actually infected with the virus, the dosage of the drug may have been too low."
The company is therefore recommending that doctors prescribe the treatment dose and not the prevention dose if the patient has any symptoms at all.
He said the drug was still useful as a preventive at the half-dose strength and that it could be taken for up to six weeks if used for that purpose.
William M. Burns, chief executive of the Roche pharma division, said the company was ready to crank up production of the drug to 400 million individual treatment packages if needed as the pandemic virus is expected to spread in the Northern Hemisphere this winter.
Another Roche official, Catherine Steele, said the company has been able to extend the shelf life of Tamiflu stockpiles to seven years from five, and it has also developed a way to extract the active ingredient from expiring stockpiles and reprocess them into new capsules to save money for developing countries.
By Associated Press Writer Alexander G. Higgins